Add support for bulk/batch requests (atomic:operations)

JsonApiDotNetCore v3.x provided support for bulk/batch by implementing the experimental [JSON:API Bulk Extension](https://jsonapi.org/extensions/bulk/). This extension was deprecated some time ago, so we removed the code in v4.

An official (non-experimental) specification is now available in the form of [Atomic Operations](https://jsonapi.org/ext/atomic/), which we intend to implement.